,20 



DEPTHS OF THE OCEAN 



furnished with four long setse. Each of these setae is attached 

 at the root to its fellow from the neighbouring cell, the result 

 being the formation of the chain. The terminal faces of the 

 cells are otherwise separate, so that there are openings between 

 them. In the winter and spring Chcetoceras decipiens is furnished 

 with thick cell-walls and stout setae, and the interstices between 

 the cells are quite inconsiderable (see Fig. 221,^); but in summer 

 the walls are thin and the setae extremely fine, and the openings 

 in the chain between the cells then become large, round or 

 oval gaps, which are almost as big as the cells themselves (see 

 Fig. 22\,d). Corresponding variations occur in other species 

 of Chatoce7'as, and in other diatoms, such as Biddulphia 

 aitrita. Along the arctic coasts, for instance, BiddzilpJiia has a 

 rather gross structure, and is almost cylindrical, with short 

 conical projections at the corners, but off the south of Norway 

 it has a comparatively much larger surface, and the corners 

 develop into long, slender outgrowths. 



We find a variation of a different nature in the case of 



Fig, 222. — Cell of Rhizosolenia hebetata-semispina {^^^). 



One end of the cell belongs to the typical arctic hebetata (on the right), the other 



to the Atlantic form semispina. 



Dimorphism. Rhizosoletiia hebetata. It occurs in two perfectly distinct forms, 

 that were formerly regarded as good species. The first, which 

 belongs to arctic waters, is thick-walled and gross, and is the 

 true R. hebetata. The second, R. semispina, has thinner walls 

 and is proportionately longer, and it is furnished with a long 

 hair-like point at each end. Its distribution extends over 

 practically the whole Atlantic, though it is chiefly to be found 

 in the neighbourhood of the cold currents. These two 

 " species " can originate from one another reciprocally as the 

 result of one cell-division. During the course of transition a 

 cell may be hebetata at the one end and semispina at the other 

 (see Fig. 222). Dimorphism of this kind is known, moreover, 

 in the case of other species. 



Still, in the open sea conditions of existence are compara- 

 tively uniform compared with what we find in coastal waters, 

 where the temperature and salinity vary considerably. Most 

 of the diatoms which belong particularly to the coastal waters 

 Resting- have a special adaptation, the so-called resting -spores, which 

 spores. must be regarded as a means of protection against such altered 



conditions. The contents of the cell can shrink into a denser 



